The operation of a power amplifier, such as used to amplify wireless RF signals for transmission, largely depends on the load impedance of the amplifier. For example, a small load impedance may result in a large output power at the expense of a large output current, which translates into a low efficiency. Contrastingly, a large load impedance may result in less output power and current consumption, and thus have a higher efficiency. Because the amplifier load impedance directly affects the output power, it plays a critical role in a transmitter's ability to meet the strict output power requirements stipulated by many governing bodies. For example, regulations governed by the FCC (Federal Communications Commission) and/or ETSI (European Telecommunications Standards Institute) generally limit the maximum output power, while performance requirements (e.g., link loss, throughput, and the like) generally limit the minimum output power. To keep the transmitter output power within the desired range, the output power of the power amplifier in the transmitter is calibrated. Typical target accuracies are +/−1 dB.
Conventional calibration techniques, e.g., factory and “on chip” calibration techniques generally assume the transmitter load is at some predetermined and fixed impedance, e.g., 50Ω, even after application of one or more external elements. However, not all customer devices operate with that load. A mismatch between the actual transmitter load impedance and the predetermined load impedance degrades the accuracy of the calibration process. Thus, there remains a need for improved transmitter output power calibration techniques.